1. Field of the Invention
The present invention relates to a method of searching record information on an information record medium such as an optical disc or the like.
2. Description of the Related Art
In the technical field of an optical disc such as a CD (Compact Disc), a DVD or the like, there is a problem that an angle between an optical axis of a light beam emitted by an optical pickup and a surface of the optical disc at an irradiation position (i.e., a tilt angle) is shifted from a perpendicular angle, due to a distortion or a surface deflection of the optical disc. The tilt angle is generated mainly in a radial direction of the optical disc and becomes a factor of a coma aberration of the optical system, which gives a harmful influence onto a reproduction quality of the optical disc since it causes a cross talk between adjacent tracks and a deterioration of a jitter. Further, if a high density recording is attempted such as a DVD, since the spot diameter of the laser beam becomes small, it is necessary to shorten the wavelength .lambda. of the laser beam and enlarge a numerical aperture of an objective lens, so that a margin with respect to the tilt angle becomes small. Namely, even if the optical disc is tilted in a very small degree, a large deterioration in the reproduction quality is generated. Therefore, in order to correct the coma aberration due to the tilt angle during the reproducing operation of the optical disc, there are installed (i) a tilt sensor including a light source of an exclusive light beam to detect the tilt angle and a detector to detect the exclusive light beam, and (ii) a device for correcting the tilt error in correspondence with the detected tilt angle, so as to apply the tilt servo control.
On the other hand, the reproducing operation of the optical disc player may not be continuously performed along a record track of the optical disc, but may have a so-called "search function" that the optical pickup is moved to a desired position on the optical disc in correspondence with an operation by a user etc., and the searching operation and the reproducing operation are alternatively repeated for each occasion. When the searching operation is performed, it is necessary to control the tilt servo control to keep up with the reproducing operation at a search target position.
By the way, the tilt angle generally changes in the radial direction of the optical disc. Thus, the values of the tilt angles at positions on the optical disc radially distant from each other e.g., the value of the tilt angle at the inner circumference side of the optical disc and that at the outer circumference side of the optical disc are drastically different from each other. Therefore, in case of performing a searching operation for a target which is positioned radially distant from the current position, the tilt angle is drastically changed. Accordingly, the tilt servo setting time is relatively long for such a radially distant searching operation, so that it takes a relatively long time to stabilize the tilt servo after actuating the tilt servo. This leads to a certain limit of the searching speed and a certain deterioration in the signal reproduction quality due to the unstability of the tilt servo at the time of starting the searching operation.
Further, since the characteristic of the tilt angle of the optical disc varies from the optical disc to the optical disc, it is difficult to estimate the tilt angle in advance. Therefore, in the information reproducing apparatus or the information recording apparatus, there is a problem that it is difficult to perform the searching operation, which search speed is high and which does not cause the deterioration in the reproduction signal quality after searching.